As development of alternative clean energy has attracted attention in the face of environmental issues and high oil prices, extensive studies have been conducted to develop improved solar cells. Organic solar cells have various advantages since they can be made slim and thin due to a high absorption coefficient of organic molecules used for photoactive layers, require a simple manufacturing process and low equipment cost, and are applicable to various fields due to good bending property and processibility of polymers. However, since electric charges have short lifetime, low mobility and a short spreading distance due to high charge trap density, organic solar cells have low light collection efficiency and low energy conversion efficiency.
Since energy conversion efficiency is an important factor in determining performance and price of a solar cell, a variety of technologies have been developed to improve efficiency of solar cells in view of various aspects. In one method of realizing a solar cell having high energy conversion efficiency, two or more unit cells each having a photoactive layer are connected in parallel to increase photocurrent while maintaining open-circuit voltage.
Representative solar cells having unit cells connected in parallel are reported by A. Hadipour et al. [J. Appl. Phys. 102, 074506] and V. Shrotriya et al. [Appl. Phys. Lett 88, 064104]. However, such solar cells require several high-degree vacuum depositions to form metal electrodes in each unit cell and employ a process of connecting the electrodes in parallel through an external wire. In this case, the unit cells can have increased photocurrent, but require complicated processes and long time in manufacture, thereby causing an increase in manufacturing costs of unit cells.